1. Statement of the Technical Field
The inventive arrangements relate to systems for positioning or pointing reflectors on a dynamic or moving basis, and more particularly to reflector assemblies that are used to focus and direct energy to and from sensors used in applications such as satellite communications and optical sensing.
2. Description of the Related Art
Directive sensors, i.e., sensors whose functionality is dependent upon the relative orientation thereof, are often used in applications that require hemispherical pointing and focusing of the energy being directed to or from the sensor, hereinafter referred to as “the sensor beam.” A passive reflector in the form of a lens and an attached mirror, hereinafter referred to collectively as a lens/mirror assembly, can be used to focus and direct the sensor beam. To effectuate hemispherical pointing of the sensor beam, the lens/mirror assembly needs to be movable so that its azimuth angle, i.e., compass direction, and elevation angle, i.e., the angle above the horizon, can be varied. Hemispherical pointing thus requires that the lens/mirror assembly rotate about at least two axes.
Lens/mirror assemblies, and other types of reflectors that require hemispherical pointing can be mounted on devices that facilitate movement of the lens/mirror assembly about a vertically-oriented axis and a horizontally-oriented axis. For example, the lens/mirror assembly can be suspended from a yoke or other type of mount that permits the lens/mirror assembly to pivot about the horizontally-oriented axis. The yoke, in turn, can be mounted on a base or other component that permits the yoke and the lens/mirror assembly, hereinafter referred to collectively as a yoke assembly, to rotate about the vertically-oriented axis.
Components such as electric motors can be used to impart motion to rotate the yoke assembly about the vertical axis and the lens/mirror assembly about the horizontal axis. The azimuth motor, i.e., the motor that causes the yoke assembly to rotate about the vertical axis, can be mounted below the base or turntable of the hemispherical pointing device, on the non-rotating portion of the device. The elevation motor, i.e., the motor that causes the lens/mirror assembly to pivot about the horizontal axis, is typically mounted on the rotating yoke. Power for the motor can be supplied by electric cabling. The use of cabling, however, can prevent the yoke assembly from being able to rotate continuously about the vertical axis. In practice, it may be necessary to reverse the direction of rotation of the yoke assembly after these components have rotated through a net angular displacement of approximately 200°, to avoid stretching and potentially damaging the cabling. The need to reverse the direction of rotation in this manner can add to the time needed for the system to respond to required changes in the azimuth angle, and can reduce the duty cycle of the motor and other components of the system that effectuate rotation of the yoke assembly.
Brush-type electrical slip rings can be used to transfer electrical power between the rotating and non-rotating components of a hemispherical pointing device, thereby eliminating the need for cabling to transfer power. Although slip rings can thereby facilitate continuous rotation of the yoke and the lens/mirror assembly about their vertical axes, slip rings typically wear and require replacement at relatively frequent intervals due to the friction inherent in the operation thereof. Slip rings can also be susceptible to the degrading effects of corrosion, particularly in seaborne applications. Moreover, the initial cost of slip rings can be relatively high.
In applications where the sensor is an active element that receives and/or transmits electrical signals and the sensor is mounted to the yoke, the signals need to be transferred between the rotating and non-rotating components of the hemispherical pointing device. Such signal transfer can be effectuated using RF rotary joints. The insertion of RF rotary joints in the signal paths, however, can introduce undesirable power losses in the signals.